Tetrazolium compounds generally are used as a redox dye (a color-developing substrate), a reducing agent, or the like. To this end, a liquid reagent prepared by dissolving a tetrazolium compound in water usually is used.
However, in the case where the pH of the tetrazolium compound solution is close to neutral, the tetrazolium compound exhibits low stability. Thus, when the tetrazolium compound solution is stored for a long time, there arises a problem in that the solution may be colored due to spontaneous color development of the tetrazolium compound or that the solution may no longer act as a reducing agent, for example. On this account, when a tetrazolium compound solution needs to be used, there is no other way but to prepare a tetrazolium compound solution for each use or to use a tetrazolium compound solution stored as an acid solution because the tetrazolium compound is stable even in the form of a solution under acidic conditions.